Rising temperatures and increased humidity are making marathons more challenging and potentially dangerous for runners. High heat and humidity hinder the body's ability to cool down, increasing the risk of heat-related illnesses. For example, the Los Angeles Marathon in 2025 saw warm conditions that led organizers to offer a shorter finish option for participants, with some runners requiring medical attention. Studies suggest that by 2045, optimal running conditions will be less likely for 46 of the 51 U.S. marathons analyzed. This trend not only affects professional athletes but also recreational runners, who may face greater health risks during races.